Electrical power distribution typically includes multiple-conductor wires to transmit electrical energy and facilitate a ground path for safety. A shock hazard exists in the event of an unintended path from the conductor wires or surfaces (such as a chassis of electrical equipment), which carry electric current, and the ground path. The conductors, such as a line conductor (also referred to as “hot”) and a neutral, or common, conductor, may leak electrical current to each other, to ground, and/or to a person or object as an intermediate path to ground. As such, a person in the intermediate path may receive a lethal electrical shock.
Ground fault circuit interrupters (GFCIs) may minimize and/or eliminate the risk of electrical shock by monitoring an imbalance of electrical current between the hot and neutral lines. Generally speaking, a line-to-ground fault may be detected by way of a coil (e.g., a toroidal Hall-effect coil) around the line and neutral conductors that provide electrical energy to a load. Under non-fault operating conditions, the magnetic fields that result from current in the hot conductor cancel with the magnetic fields that result from an opposite current flow in the neutral conductor, thereby failing to induce a corresponding electrical current in the coil. However, if current from the line conductor leaks current to ground, then the neutral conductor current, and its corresponding magnetic field, will be less than the magnetic field of the line conductor, thereby affecting the coil to produce a corresponding electrical signal indicating a fault. The electrical coil signal, such as a current value, may be compared to a threshold which, when exceeded, causes the GFCI to force a mechanical break to the load via, for example, a circuit interrupter. The circuit interrupter may be employed as a double pole, single throw switch that, when activated, physically separates the line and the neutral conductors from the load.
Detecting a neutral-to-ground fault poses additional challenges because, in part, the neutral conductor is also grounded at the source. Such double grounding of the neutral conductor could create a situation where a portion of the fault current from the line conductor returns to the source through the neutral conductor. As a consequence, the traditional single coil approach will not detect a flux imbalance representative of the actual current leakage magnitude. To aid in neutral-to-ground fault detection, a second coil is typically employed that, when coupled to the first coil, produces a positive feedback loop. Despite the lower detected current imbalances observed during a neutral-to-ground fault, which may not exceed a tripping threshold, the coupled coils will develop an oscillation that, when detected, may be used to indicate a circuit trip or interruption is warranted. Additionally or alternatively, a signal may be injected on the second coil so that, in the event of a neutral to ground fault, the injected signal is induced in the neutral line and is detected by the first coil.